Water Transport in the Gas Diffusion Layer of a Polymer Electrolyte Fuel Cell: Dynamic Pore-Network Modeling

Abstract

The pore-scale modeling is a powerful tool for increasing our understanding of water transport in the fibrous gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). In this work, a new dynamic pore-network model for air-water flow in the GDL is developed. It incorporates water vapor transport coupled with liquid water by a phase change model. One important feature of our pore-network model is that a recently developed semi-implicit scheme for the update of water saturation is used. It provides good numerical stability in modeling liquid water transport in the GDL even for very small capillary number values. A number of case studies are conducted to illustrate several important mechanisms of water transport in the GDL, such as cyclic processes of local drainage and imbibition, channeling and capillary-fingering evolutions of water flow pattern, and eruptive water transport. Moreover, we show that liquid water separation in the GDL between the ribs and gas channel (GC) is formed under dry GC condition, which is qualitatively in agreement with in situ experimental results. Water management plays an important role in the durability and performance of polymer electrolyte fuel cells (PEFCs). A delicate water balance must be maintained to keep the membrane hydrated and at the same time avoid liquid water flooding in the porous components. [1][2][3] Over the past two decades, numerous studies have been conducted for improving our understanding of water transport mechanisms within the PEFC. Compared to the anode side, water transport in the cathode has usually attracted more attention. This is because the cathode is prone to being flooded first, and the oxygen reduction reaction (ORR) is very sluggish. The fibrous gas diffusion layer (GDL), as a key component, plays an important role in removing excessive water from the catalyst layer (CL) to the gas channel (GC). Up to now, much effort has been invested in gaining insights into pore-scale processes of liquid water transport in the GDL. [4][5][6][7][8][9][10] Despite the great success in in situ observations of water distribution, 8,9,[11][12][13][14] water dynamics in the GDL is still not well understood. Also, the cost of those experiments is quite high. This has stimulated many pore-scale numerical studies of the GDL over the past few years. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Several distinctive features of liquid water transport in the GDL make its pore-scale modeling extremely difficult. First, a small flow rate of liquid water, with a capillary number value of around 10 −8 , causes the modeling computationally expensive. Second, the complexity 34 of the pore structure and mixed wettability of the GDL give rise to the difficulty in developing an elegant pore-scale model. Nevertheless, many achievements still have been booked.Mukherjee et al. 4 were the first to conduct the Lattice-Boltzmann modeling of air-water flow in a stochastically reconstructed nonwoven GDL. The authors illustrated the structure-wettabili...

Liquid water in cathode gas diffusion layers of PEM fuel cells: Identification of various pore filling regimes from pore network simulations

“…This has motivated quite a few studies using various techniques, such as Lattice Boltzmann Method, e.g. [11], or pore network models (PNM) [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. More classical continuum approaches based on the standard twophase flow model for porous media have been also used, e.g.…”

Experimental Investigation of Hysteretic Dynamic Capillarity Effect in Unsaturated Flow

“…Cycles of imbibition and drainage (main or scanning) may occur in many field applications, such as irrigation, groundwater recharge, infiltration and redistribution in vadose zone, and root uptake processes. They are also encountered in industrial applications, such penetration of ink into paper and its subsequent drying (see, e.g., Aslannejad et al, ), redistribution of moisture in layers of fuel cell (see, e.g., Qin, ), and infiltration and drainage of liquids in hygienic products such as diapers. Also, investigation of dynamic scanning drainage or imbibition will give a full insight into the dynamic capillarity coefficient over the entire hysteretic loops in a porous medium system.…”

Pore Network Modeling of Compressed Fuel Cell Components with OpenPNM

“…However, under non‐isothermal conditions with flowing gasses such as those typically encountered within a fuel cell, the liquid may condense under the cooler rib sections and evaporate where convection is strongest under the channels 40, 41. Two recent pore network studies that investigated phase change phenomena support this hypothesis, but did not account for porosity variation or local pore structure changes due to compression 42, 43. Further work is required to assess the relative importance of the competing transport effects on the liquid percolation under different operating fuel cell conditions.…”